The present invention relates to a color cathode ray tube and, particularly, to a color cathode ray tube having an electron gun that makes it possible to obtain favorable focusing characteristics over the whole fluorescent screen and that executes efficient speed modulation.
A technique for improving the picture quality of TV receivers and computer monitors can be represented by a method disclosed in Japanese Patent Laid-open No. 140428/1976, in which the scanning speed of the electron beam is modulated with a brightness-changing portion of the picture (or the image) to emphasize the contour of the picture. This method is generally called speed modulation.
Such speed modulation includes both an electromagnetic type and an electrostatic type. The speed modulation of the electromagnetic type, however, has been more generally used. The speed modulation of the electromagnetic type is produced by an electromagnetic coil attached around the neck portion of a cathode ray tube and a circuit for driving the electromagnetic coil.
FIG. 4 is a schematic sectional view illustrating a color cathode ray tube of the speed modulation type. In this color cathode ray tube, a vacuum enclosure is constituted by a panel portion 20, a neck portion 21 and a funnel portion 22. On the inner surface of the panel portion 20 there is formed a fluorescent screen 23 by arranging fluorescent materials of three colors in the form of a mosaic or stripes, and adjacent the back surface of the fluorescent screen 23 there is provided a shadow mask 24 which operates as a color-selection electrode. The shadow mask is held by a mask frame 25 and is supported together with a magnetic shield 26 on the inner surface of a skirt of the panel portion by a mask suspension mechanism 27. Furthermore, an electron gun 28 of an in-line type is contained in the neck portion 21, and a deflecting device 29 is so provided as to surround a transition region between the neck portion 21 and the funnel portion 22.
Reference numeral 30 denotes a magnetic device for correcting the color convergence and the color purity of the electron beam. An electromagnetic coil 34 for speed modulation is further provided around the neck portion 21.
Reference numeral 31 denotes stem pins for feeding image signals and various drive voltages to the electrodes of the electron gun, 32 denotes a getter for increasing the degree of vacuum, and 33 denotes a band for preventing implosion.
An electron beam B emitted from the electron gun is subjected to speed modulation by the magnetic field generated by the electromagnetic coil 34 surrounding the neck portion 21 to reproduce an image having a high picture quality on the fluorescent screen 23.
FIGS. 5A and 5B are diagrams illustrating the constitution of a conventional electron gun used for the color cathode ray tube shown in FIG. 4. FIG. 5A is a side view and FIG. 5B is an end view as seen in the direction indicated by arrow P in FIG. 5A.
The electron gun is constituted by a cathode 1, a first electrode 2, a second electrode 3, a third electrode 4, a fourth electrode 5, a fifth electrode (focusing electrode) 6, a sixth electrode (anode) 7, and a shield cup 8. Reference numeral 9 denotes bead glass for firmly holding the electrodes, 10 denotes a stem, and 11 denotes contact springs.
A shield cup 8 is connected to the anode 7 on the fluorescent screen side. Referring to FIG. 5B, openings 81, 82 and 83 are formed in line in the bottom of the shield cup 8 for the three electron beams.
The focusing electrode 6 is divided into two parts, i.e., a first division electrode 61 and a second division electrode 62, in the axial direction of the tube. The whole electrode has a length L in the axial direction of the tube, the first division electrode 61 having a length L1 in the axial direction of the tube, and the second division electrode 62 having a length L.sub.2 in the axial direction of the tube, satisfying the relation L.sub.1 .ltoreq.L.sub.2.
FIGS. 6A and 6B are diagrams illustrating the constitution of the second division electrode constituting the focusing electrode of FIG. 5A. FIG. 6A is a front view as viewed from the anode 7 side, and FIG. 6B is a side view showing a portion thereof in cross section.
Openings 62a, 62b and 62c are formed in the second division electrode 62 on the first division electrode 61 side for the respective electron beams. A single opening 62d is formed in the surface thereof opposed to the anode, and it has a diameter D in a direction at right angles with the in-line direction. In the electrode there are further installed an inner electrode 64 and a plate-like correction electrode 63 having openings for the respective electron beams. Reference numeral 65 denotes tabs buried in the bead glass.
An electron gun having the above-mentioned electrode constitution is disclosed in Japanese Patent Laid-open Nos. 103752/1983 and 152834/1992.
The second division electrode 62 has a length L.sub.2 in the axial direction of the tube; the length from the surface thereof opposed to the first division electrode 61 to the electron beam passing opening in the inner electrode 64 is L21 in the axial direction of the tube; the length of the surface thereof opposed to the anode 7 to the electron beam passing opening in the inner electrode 64 is L.sub.22 in the axial direction of the tube; and the length from the surface thereof opposed to the anode 7 to the electron beam passing opening in the plate-like correction electrode 63 is L.sub.23 in the axial direction of the tube. Here, the length L.sub.21 is L.sub.21 .gtoreq.L.sub.22.
The main lens of the electron gun is formed in a portion shown in FIG. 5A, where the anode 7 and the focusing electrode 6 are opposed to each other and the focusing electrode 6 is divided into two parts, i.e., the first division electrode 61 and the second division electrode 62 in the axial direction. The electric field produced by the electromagnetic coil surrounding the neck portion enters the electrodes through gaps among the main lens-forming portion, the first division electrode 61 and the second division electrode 62, and the electron beam passing through the main lens-forming portion and the two division electrodes is temporarily deflected by the magnetic field to control the scanning speed of the electron beam, i.e., to effect a so-called speed modulation.
The magnetic field generated by the electromagnetic coil forms eddy currents in the electrodes of the electron gun to suppress the action of speed modulation.
In order that the action of speed modulation is not suppressed by the eddy currents produced in the electrodes of the electron gun due to the magnetic field established by the electromagnetic coil, Japanese Patent Laid-open No. 146847/1980 discloses a method according to which a slit of a relatively broad width is formed in the electrodes of the electron gun at a position where the electromagnetic coil is installed.
In the above described device, however, the electrodes for speed modulation are formed in the shape of a relatively deep box, and the gap (slit) formed between the electrodes is located relatively far away from the position of the main lens (focusing gap) toward the cathode. From the standpoint of the overall length of the electron gun, as well as the cathode ray tube, however, this structural arrangement is not suited for shortening the depth of current TVs and monitors. Besides, no consideration has been given at all concerning the efficiency of speed modulation.